Medical staff use a Geiger counter to screen a woman for possible radiation exposure at a public welfare centre in Hitachi City, Ibaraki, March 16.(Reuters)

Japan put its top priority on efforts to cool down a plutonium-fuelled nuclear reactor, attempting at one stage to water-bomb the facility without success.

Japan put its top priority on Wednesday on efforts to cool down a plutonium-fuelled nuclear reactor, attempting at one stage to water-bomb the facility without success amid fears that authorities were running out of options to avert disaster.

The No.3 reactor is the only one of the quake-stricken Fukushima Daiichi plant's six reactor units to be powered with plutonium, which is far more hazardous to health than uranium, which is used to power the other five reactors.

High radiation levels had stopped a military helicopter from dumping water on the No.3 reactor, local media said. The plant's owner, Tokyo Electric Power , declined to comment.

It was not clear if the helicopter was trying to pour water on the reactor vessel or on an associated spent-fuel pool, a deep reservoir where spent nuclear fuel rods are kept. Public broadcaster NHK said the water was for the spent-fuel pool.

Later, Kyodo news agency quoted a government taskforce as saying it was unlikely the No.3 reactor had sustained major damage.

The number of workers at the complex has increased to 180, up from 50 a day earlier but still far less than the 800 that were deployed there immediately after Friday's devastating 9.0 magnitude quake and tsunami.

Debris on roads has hampering efforts to bring in equipment, and there are plans to bulldoze a road into another part of the complex, the No.4 reactor, which is the second priority.

"The situation at the No.4 reactor is not exactly a good situation but the No.3 reactor is a higher priority," a Tokyo Electric Power official told a media briefing.

On Tuesday, an explosion blew two holes in the wall of a building housing the No.4 reactor unit's spent-fuel pool, exposing it to the atmosphere.

Tokyo Electric also said it was not sure what was happening at the No.2 reactor, which saw a sudden drop in pressure.

The company said this might indicate it was being cooled or it could also be an ominous sign that there was a hole in the core container. Or it might just be a faulty pressure reading.

"We cannot with certainty say whether or not the core container is airtight," the company official said.

The company also said it was pouring water at the reactors No.5 and No. 6, but it was also not immediately clear into which parts of those two reactor units.

It did say, however, that temperatures at the spent-fuel pools in those reactors had risen slightly since Wednesday morning. There was water in the pools but it was having trouble circulating the water because of cooling-system problems.

There was no new data on temperatures of the spent- fuel pools at reactors No.1 to No.4.